Integrated circuit dies include contact pads, or input/output pads, along edges of the dies to allow electrical communication between circuitry within the dies and outside circuitry. Lead frames have a plurality of conductive fingers attached to the contact pads either directly or by means of other conductors such as bond wires. A die and the inner lead ends of the conductive fingers are typically encapsulated within ceramic or other dielectric material. Mechanical and electrical connection to outside circuitry is made at portions of the conductive fingers outside of the encapsulation. For example, the outer lead ends may be soldered to a printed circuit board or the like.
Alignment bars are utilized to link adjacent conductive fingers during the fabrication and attachment steps. The most commonly known alignment bars are tie bars and dam bars. As described in U.S. Pat. No. 4,987,475 to Schlesinger et al., dam bars interconnect the conductive fingers in the plane of the lead frame and act as dams which limit the flow of material during transfer molding of the encapsulation. Schlesinger et al. describe one molding process as joining two mold halves together with the conductive fingers projecting through an opening between the halves, thereby leaving gaps between the conductive fingers and the mold halves through which the encapsulation material can extrude during molding. The dam bars prevent any significant flow of material. Tie bars connect the conductive fingers at the outer lead ends. During the molding process, the tie bars serve to maintain the proper alignment and spacing of the leads.
Dam bars and tie bars are typically made of the same material as the conductive fingers, e.g., copper. The lead frame is fabricated so that the bars and the conductive fingers are a unitary structure. The bars electrically short the conductive fingers together Thus, the bars must be removed after the intended function has been fulfilled. Following encapsulation of the integrated circuit die, the dam bars are removed by a punching operation. Tie bars are removed by cutting or shearing the bars from the conductive fingers.
The punching operation for removing bars presents a number of difficulties. With the ever-present goal of miniaturizing electronic components, integrated circuit dies are decreasing in size and increasing in the number of contact pads. Consequently, the density of conductive fingers is increasing. The punching operation of bars imposes a manufacturing limit on density. Reliably punching each portion of a dam bar from between adjacent fingers becomes more difficult as density increases. Schlesinger et al. describe the difficulties of removing tie bars as the potential of leaving "pigtails" of metal and as adding handling steps.
Other alignment bars are known. Schlesinger et al. teach use of a web bar that is integral with the conductive fingers and that provides enhanced lead alignment when the conductive fingers are brazed to pads of fine line ceramic integrated circuit packages. After brazing, the web bar is mechanically removed using a punch trim tool. Tooling holes in the lead frame provide alignment for the punch trim tool, thereby increasing the accuracy and reliability of bar removal U.S. Pat. No. 4,796,080 to Phy teaches use of intermediate bars located between tie bars and dam bars. The intermediate bars are made of a semiconductor material to guard against damage caused by electrostatic discharge during handling, but to provide sufficient electrical isolation of conductive fingers to allow testing prior to removal of the intermediate bars. The dam bars and tie bars are removed using conventional steps, whereafter the alignment and separation of fingers is maintained by the semiconductor intermediate bars. Removal of the intermediate bars is facilitated by scoring, or by localized stressing or weakening, the conductive fingers. U.S. Pat. No. 4,801,561 to Sankhagowit teaches use of cross-links that function as dam bars to prevent ejection of encapsulent from the molding process between the conductive fingers. The cross-links are integral with the conductive fingers and are removed in a manner described above.
It is an object of the present invention to provide a method of fabricating a lead frame inner connection assembly having alignment bars which allow removal without imposing manufacturing limits on the density of leads of a lead frame.